Foamed metal

ABSTRACT

A PROCESS FOR STRENGTHENING A FOAMED METAL OR FOAMED METAL ALLOY WITH A MEAL OXIDE FORMED IN SITU BY MELTING THE METAL OR METAL ALLOY, MIXING AN OXYEN CONTAINING AGENT INTO THE METAL OR METAL ALLOY, AND FOAMING THE METAL OR METAL ALLOY.

United States Patent O 3,705,030 FOAMED METAL Currie B. Berry, Jr., and Robert J. Fanning, Baton Rouge, La., assignors to Ethyl Corporation, New York, N.Y. No Drawing. Filed Oct. 30, 1970, Ser. No. 85,783 Int. Cl. BZZd 25/00, 25/06 US. Cl. 75-20 F 5 Claims ABSTRACT OF THE DISCLOSURE A process for strengthening a foamed metal or foamed metal alloy with a metal oxide formed in situ by melting the metal or metal alloy, mixing an oxygen containing agent into the metal or metal alloy, and foaming the metal or metal alloy.

BACKGROUND OF THE INVENTION In the production of foamed metal, this is, metal having a plurality of randomly dispersed closed cells throughout a metal matrix, a preferred method is to use a heat decomposable foaming agent to genate the gas to form the cells. This technique is disclosed in Us. Pats. 2,751,- 289; 2,895,819; 2,983,597; 3,300,296; and 3,297,431.

Such prior art foams frequently have cells which are of non-uniform structure or undesirable large size. This problem has been to some extent alleviated by increasing the viscosity of the molten metal with various viscosity increasing agents to aid in the subsequent blowing step. Decreased fluidity, i.e. thickening, of the molten metal enables the foaming operation to be relatively prolonged and the foamed metal to be maintained in its heated, fluid condition without collapsing for relatively prolonged periods since the trapped bubbles cannot readily escape from the thickened melt.

It has now been discovered that the use of certain thickening agents also causes the foamed product to be strengthened. Of course, the amount of agent required for optimum thickening may not correspond to the amount of agent required for increased strengthening. Therefore the present invention is directed not only to a foam strengthening process employing these certain agents but also to providing a method for controlling the amount of agent in order to provide the described strengthening. The following description of the invention will demostrate how this in accomplished.

SUMMARY OF THE INVENTION The present invention provides a process for strengthening a foamed metal or foamed metal alloy with a metal oxide formed in situ by melting the metal or metal alloy, mixing an oxygen containing agent into the metal or metal alloy, and foaming the metal or metal alloy.

The present invention also provides, in the process of foaming a metal or metal alloy wherein the metal or metal alloy is melted, thickened with metal oxide formed in situ and foamed, the improvement of adjusting the amount of metal oxide by measuring the effect of the oxide on the strength of the foamed metal or metal alloy.

In accordance with the above described procedure, the present invention easily achieves the previously mentioned objective, i.e. a foam metal having improved strength characteristics is produced without sacrifice of other favorable characteristics such as uniform cellular structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Among the metals suitable for use in this invention, such as aluminum, magnesium, titanium and the like,

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aluminum is preferred. Aluminum alloys are highly preferred and especially desirable are alloys of about 96 to about percent by weight aluminum with about 4 to about 15 percent by weight magnesium.

The first step in the practice of the process of this invention is to melt the metal or its alloy. This may be achieved with any suitable apparatus well known in the art. It is desirable that such apparatus be susceptible to being maintained under an inert gas purge. Gases such as nitrogen are especialy suitable for this purge. If a melting pot is employed, the pot is raised to a temperature well above the temperature at which the metal or its alloy normally becomes a liquid. This facilitates quickly melting the metal or its alloy inasmuch as the temperature is allowed to slowly recede as the metal or its alloy becomes completely liquid. Desirably the temperature of the metal or its alloy is stabilized to the point of about 10 F. to about 50 F. above its melting point in order to insure that it Will remain liquid during the remainder of the process.

Once the metal or its alloy has been liquified, it is agitated or stirred by means known in the art, e.g. an impeller, propeller, turbine, or the like. Where a turbine is employed, a rotation rate of 100 r.p.m. to 10,000 r.p.m. is suitable although 3,000 r.p.m. to 6,000 r.p.m. is generally preferred. It also appears useful to employ a slower rate of stirring before the foaming agent. described hereinbelow, is added to the molten metal or its alloy.

Once stirring or agitation of the molten metal or its alloy is well under Way, the viscosity-strength increasing agent is added to the molten material. Viscosity-strength increasing agents of this invention include air, oxygen, and carbon dioxide, among which oxygen is preferred. Not all viscosity increasing agents also function as strength increasing agents. For example, nitrogen and argon are thickening agents which do not co-function as strengthening agents. In order to increase strength, the agent must contain oxygen and form oxide particles within the melted metal. It is critical that such metal oxide be formed in situ; thus, adding preformed metal oxide to the melted metal will not increase strength to any apparent or sufiicient degree although such oxide will increase viscosity. The mechanism by which the strength is improved through oxide formed in situ is not known.

The physical state of the viscosity-strength increasing agent is not critical inasmuch as these compounds can be employed as solids, liquids, or gases. Thus, the thickeningstrengthening agent may be employed in the physical state which is most convenient which in the case of car- 'bon dioxide may be in solid form and in the case of oxygen in the gaseous form.

On the other hand, it is critical that the viscositystrength increasing agent be uniformly admixed within the molten metal or its alloy. Thus, it is not enough to treat only the surface of the molten material. The thickening-strengthening agent must be blended uniformly into the molten material by agitation or stirring at the rates above described.

Improvement in strength of the foamed metal product usually requires the use of more thickening agent than does optimum increase in viscosity of the molten metal. However, the present invention does not contemplate what would normally be considered substantially overthickening the melt, i.e. thickening the melt to an extent such that the thickness substantially adversely interferes with the foaming process. Accordingly, with this limitation as to over-thickening, it is suitable to use any quantity of thickening-strengthening agent which meets the purposes of this invention, this quantity being of course dependent upon the particular thiokening agent selected, the particular metal or metal alloy being foamed, process conditions, and the type of apparatus employed for the process. Generally, it is suitable to use a quantity of thickening-strengthening agent within the range of from about 0.5 pound of oxygen per 100 pounds of metal or agent per 100 pounds of the metal or its alloy and preferable to use from about 0.1 to about 1 pound of agent per 100 pounds of the metal or its alloy. More specifically, it is suitable to use from about 0.02 pound or less to about 1 pound or more of oxygen per 100 pounds of metal or its alloy, preferable to use from about 0.07 to about 0.5 pound of oxygen per 100 pounds of metal or its alloy, and more preferable to use from about 0.1 to about 0.2 pound of oxygen per 100 pounds of metal or its alloy. It is suitable to use from about 0.2 pound or less to about pounds or more of air per 100 pounds of metal or its alloy, preferable to use from about 0.7 to about 5 pounds of air per 100 pounds of metal or its alloy, and more preferable to use from about 1 to about 2 pounds of air per 100 pounds of metal or its alloy. It is suitable to use from about 0.05 pound or less to about 4 pounds or more of carbon dioxide per 100 pounds of metal or its alloy, preferable to use from about 0.1 to about 1 pound of carbon dioxide per 100 pounds of metal or its alloy, and more preferable to use from about 0.3 to about 0.7 pound of carbon dioxide per 100 pounds of metal or its alloy.

Preferably, the viscosity-strength increasing agent is added to the molten material at a rapid rate. The time period of addition may vary from about 5 seconds to about minutes and is subject to lengthening where very large quantities of thickening agent are employed.

Generally, the pressure at which the viscosity-strength increasing agent is added to the molten metal or its alloy is not highly significant. Subatrnospheric, super-atmospheric or ambient pressures can be used although ambient pressure is preferred for reasons of economics. However, high pressure may tend to favorably force a gaseous agent into the molten metal or its alloy while the associated closed vessel retards escape of the gaseous agent.

After the metal or its alloy has been subjected to addition of the strengthening agent, it is adjusted to a proper temperature for foaming. A wide variety of blowing agents can be used in the foaming process of this invention. Broadly, all blowing agents described in the prior art are suitable although some blowing agents are better than others. However, Whatever the blowing agent, the foams of this invention have smaller, more uniform pore size than foams produced from the same metal substrate which have not been made more viscous by the thickening-strengthening agents above described.

Among the various blowing agents, the metal hydrides are preferred, among which titanium, hafnium or zirconium hydrides, especially the latter, are most preferred. Dihydrides and annealed hydrides of less than stoichiometric composition also can be employed. Generally, the best hydride blowing agents are those which decompose to yield gaseous hydrogen at the temperature of the metal or its alloy which is to be foamed and release hydrogen at a relatively slow rate.

The amount of foaming desired determines the amount of hydride or other blowing agent employed; that is, for a dense foam less blowing agent is used than for a lighter foam. Usually, it is preferred to make foams having a percent density or less, or to make foams which weigh no more than about 20 percent of the weight per given volume of the unexpanded metal. For such foams it is suitable to employ from about 0.5 to about 2 pounds of hafnium hydride, titanium hydride, or zirconium hydride for each 100 pounds of molten metal or its alloy to be foamed. A preferred range is from about 0.6 to about 1.2 pounds per 100 pounds of molten metal to be foamed.

In the foaming step, a temperature is employed which is above that at which the metal or its alloy to be expanded is molten and above the temperature required to thermally decompose the blowing agent. The temperature, however, must not be so high that the blowing gas is released so fast as to cause foaming at an uncontrollable rate. Thus it is preferred to have the temperature of the molten metal or its alloy comparatively cool. Taking all these factors into consideration, a typical aluminummagnesium alloy is foamed at temperatures within the range from about l,l30 F. to about 1,250 F. and preferably from about 1,150 F. to about 1,200 F Generally, it is suitable to carry out the foaming process at ambient pressure although greater or lesser pressures can be employed with better results under certain circumstances. Lower pressures can be deleterious since they can encourage evolution of blowing gas outside the confines of the mass to be foamed. Super-atmospheric pressures up to 1,500 p.s.i.g. or higher can be used.

In carrying out the blowing step above described, the foaming agent is preferably admixed with the molten metal or its alloy to be foamed by using the agitating or stirring means earlier set forth. In the course of such stirring or agitation, the rate is preferably increased above the initial agitation rate at which time the foaming agent is added. Without exception, the more uniform the mixing, the better the foam. All techniques of mixing known in the art which ensure efiicient mixing of materials and liquids can be employed. Preferably the mixing step is performed in as short a time as is feasible to achieve uniform mixing. For 'best results with a typical mixture of blowing agent and molten metal or its alloy, sufficient mixing achieves homogeneity within about 10 seconds. This time period may require an agitation rate with a stirring device of up to 10,000 r.p.m.

It is generally preferable that the addition of foaming agent be at a lower temperature than the addition of the viscosity-strength increasing agent. Accordingly, it is pre ferred to cool the viscous metal before adding the foaming agent. Frequently, the cooling is best carried out in a second vessel, i.e., a vessel other than the hot chamber in which viscosity and strength were increased. The second vessel is preheated to within plus or minus 50 C., preferably plus or minus 20 C. of the foaming temperature, whereupon the viscous metal or its alloy is added thereto.

Subsequent to the addition of the blowing agent, the molten metal or its alloy is allowed to foam. Foaming may occur within an open or closed mold. The size of closed foaming chambers relative to the quantity of metal or its alloy determines density of the product. Regulation of the mold temperature determines the smoothness and thickness of the skin on the finished article.

Having thus described the invention, the following examples are presented as being illustrative and not limiting of the invention.

EXAMPLE I Three successive runs are made in which one, three and five cubic feet of oxygen are vigorously agitated into two-hundred pound batches of molten aluminum alloy containing seven percent magnesium. The resulting partially oxidized and thickened alloy is then foamed by adding two pounds of zirconium hydride, agitating, and transferring into a mold. The resulting foams have tensile strengths which vary directly with the amount of oxygen employed, indicating that the more thickening agent used, the stronger the foam.

EXAMPLE II In three different runs air is bubbled into ten pounds of molten aluminum for five minutes, ten minutes, and fifteen minutes. The resulting thickened melt is foamed in the above described manner. Foams produced from these three runs are found to have increasing strength according to how much air is bubbled into the melt. The foam from the run with ten minutes of air addition is stronger than the foam from the run with five minutes of air addition, and the run with fifteen minutes of air addition is stronger than the foam from the run with ten minutes of air addition.

EXAMPLE III One-half pound of carbon dioxide as crushed Dry Ice was agitated into 250 pounds of an aluminum alloy containing five percent magnesium. The resulting molten a1- loy was moderately viscous. The alloy was then foamed in the manner herein described. The resulting foam had a density of fifteen pounds per cubic foot and a tensile strength of 200 pounds per square inch.

Subsequently, 1.5 pounds of Dry Ice were used to pretreat the same amount of the same alloy. The resulting foam had the same density and appearance, but the tensile strength was 290 pounds per square inch. The increased strength was due solely to the use of more Dry Ice.

What is claimed is:

1. A process for strengthening a foamed metal or foamed metal alloy with a metal oxide formed in situ, comprising: melting the metal or metal alloy, substantially uniformly admixing an oxygen containing viscositystrength increasing agent selected from the group consisting of oxygen, air or carbon dioxide into said metal or metal alloy, mixing a metal hydride blowing agent into said metal or metal alloy at a temperature lower than the temperature at which the viscosity strength increasing agent is added, and foaming said metal or metal alloy, thereby producing a foamed metal or metal alloy having a greater tensile strength than a foamed metal or metal alloy similarly produced but without the addition of the viscosity-strength increasing agent.

2. The process of claim 1 wherein said melted metal is aluminum and from about 0.001 to about 0.05 pound of metal oxide is formed per pound of aluminum.

3. The process of claim 1 wherein said melted metal alloy comprises from about 96 to about 85 percent by weight aluminum and from about 4 to about 15 percent by weight magnesium and from about 0.001 to about 0.05 pound of aluminum oxide is formed per pound of alloy.

4. In the process of foaming a metal or metal alloy wherein said metal or metal alloy is melted, thickened and strengthening with a viscosity-strength increasing agent selected from the group consisting of oxygen, air or carbon dioxide substantially uniformly admixed therein which forms a metal oxide in situ and foamed with a metal hydride blowing agent added to the molten metal or metal alloy at a temperature lower than the temperature at which the viscosity-strength increasing agent is added, the improvement comprising adjusting the amount of metal oxide by measuring the effect of the oxide on the tensile strength of the foamed metal or metal alloy.

5. The process of claim 1, wherein said viscositystrength increasing agent is added in an amount in excess of that necessary to produce optimum viscosity but not substantially over-thickening the molten metal or metal alloy.

References Cited UNITED STATES PATENTS 3,214,265 10/1965 Fiedler 75-20 F 3,305,902 2/ 1967 Bjorksten 75-20 F X 3,379,517 4/1968 Graper 75-20 F L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner 

